Modern paper making processes and machinery follow the Fourdrinier method wherein an aqueous dispersion of paper making fibers is poured onto a high speed travelling woven fabric through which water from the dispersion drains leaving a thin web of wet fibers which is dried and finished to a sheet of paper. The key step in this method is that of forming the web from the fiber/aqueous dispersion. This must be done very quickly and uniformly across the width of the endless fabric. Normally, the transition of dewatering commences by gravity, followed by other means such as foil blades, continuing with a plurality of controlled low vacuum boxes and then by a plurality of high vacuum boxes. There are many causes for mishaps to occur that prevent the final sheet of paper from being perfect. One of the principal causes is that air may penetrate the web of paper and the fabric causing nonuniformities in the paper. Such disturbances may be caused by nonuniform drainage at every square inch of the fabric surface, and entrainment of air in the fiber/aqueous dispersion followed by forcing air through the dispersion and fabric whereby air will find the path of least resistance and fixing the fluctuation of such dispersion unevenly over the fabric. The demand for higher and higher speed makes it increasingly difficult to produce a paper sheet that is isotropic.
The critical step of this process is the water removal, which must be done quickly and uniformly in order to obtain a layer of fibers on the fabric that can be finished to a high quality paper. The principal difficulty in producing a fast, uniform drainage has been that when the drainage is speeded up by applying a vacuum there are numerous instances at random locations across the fabric where air will be pulled through the layer of wet fibers. At each location a small vortex appears to break the continuity of the film of water and fiber on the fabric, and to permit the passage of air through the entire film and thereby disrupting the uniform settling of the fibers into a web of uniform thickness and strength. Every time such an instance occurs, a meniscus is formed at the interface of the water and air and this is an obstruction to the free uniform flow of water away from the fibers forming the web. The formation of such air holes through the mass of fibers forming the web must be minimized if any improvement in sheet formation at high speed is to be achieved.
It is an object of this invention to provide improved sheet formation in the Fourdrinier paper making process. It is another object of this invention to provide an improved procedure for maintaining a continuous drainage of water with substantially no air flow discontinuities occurring in the forming web. It is an object of this invention to provide a drainage process wherein all of the web forming fibers are essentially submerged in water until the last moment when the last portion of water is drained away from all parts of the web simultaneously. A further object is to improve the drainage while maintaining a higher retention of fines and fillers in the web than heretobefore accomplished. For example, the prior art mills may have a first pass retention of between 40-60 percent whereas this invention provides first pass retention of up to 90%. Another object is to decrease the amount of friction between the fabric and the dewatering components to increase the fabric life. A further object is to substantially reduce the length of the forming area of the Fourdrinier fabric, thereby reducing the number of dewatering components required. For example, one submerged drainage box in accord with this invention may replace 20-25 foils of the prior art and in substantially less space along the length of the fabric. An additional object is to improve the sheet formation by decreasing its porosity and substantially eliminating pin holes through the sheet. Another object is to decrease the power consumption of the Fourdrinier machine in both driving the fabric and by eliminating high vacuum pumps to supply suction to the dry end flat boxes thereof.
Yet other objects include:
A. retention of more chemical additives and fines due to the more gentle dewatering and uniformity of dewatering; PA0 B. easier release of web from the fabric due to the web not being forced into the interstices of the fabric by high vacuum whereby a web pick-up vacuum roll or high pressure air from below the fabric not needed; PA0 C. amount of defoamer is reduced; PA0 D. enhanced sheet strength; and PA0 E. enhanced drying at the drier end of the fabric (couch roll) thus reducing the power used in the press and/or the drying sections. Still other objects will appear from the more detailed description which follows. PA0 A. Meniscus is the surface area of a water volume which is in contact with unlike surfaces. The unlike surfaces being either the container holding the water or the gases in contact with a surface of the water or surrounding the water, such as air when a drop of water is falling through it. Webster's New International Dictionary, 2nd Edition, Unabridged, 1934 defines Meniscus as--the curved upper surface of a liquid column that is concave when the containing walls are wetted by the liquid and convex when not. However, the meniscus also is present at the interface between the liquid and the vessel in which it is contained. PA0 B. Surface Tension is a condition that exists at the free surface film of a liquid by reason of intermolecular forces about the individual surface molecules and is manifested by properties resembling those of an elastic skin under tension. Surface Tension is a characteristic of the water meniscus which can be modified by chemical means. The meniscus changes its geometric (concave) shape depending on the size of the vessel containing the fluid. In capillary tubes the meniscus reaches extremely high levels of energy in the form of pressure. The resistance of the meniscus to rupture, compared to it's thickness is very high as is well known. PA0 C. Draining by eliminating the meniscus, or submerged drainage, is a water removal operation whereby water is removed from the aqueous dispersion or wet web by means of a reduction of pressure originating from, and transmitted by the water and not by the prior art vacuum as may be provided in the wet and drier end of a Fourdrinier fabric. In particular, the meniscus is eliminated in the surface of the fabric opposite to the pulp or web so that drainage is unimpeded.